The easiest way to make money from apps is to earn on ads. Depending on the kind of the app you should use suitable formats. For instance if you have a game - stick to rewarded video or at least interstitials. If it's a utility app - check native ads, moreover some networks, Appodeal for example supports Native Video. The main point is to choose the right ad format, placement and frequency.
There’s a plenty of various networks: mopub , vungle, unityads, chartboost, supersonic, admob appodeal, heyzap and so on. Among of them you see several mediators like Appodeal and Heyzap that allow you to cover several networks in one account. For the last couple of years i've tried some.Now im using rewarded videos, cause they have pretty high ecpm and keep gamers loyal to ad. Tried Chartboost for videos and interstitials and really liked it, they show game related ads. Vungle and Unity Ads were also ok, but after testing them I took a look on Heyzap cause they provide multiple networks – Didn’t like it because of lower eCPM + some difficulties with integration. After that tested Supersonic, its was normal, but decided to switch on Appodeal, as they pay instantly -no net30 or net60 it makes huge sense for me. Also it has all major networks in one account. The integration was painless. Had a tech issue as i'm not a tech ninja, but their support helped me to solve the problem within an hour. 'm on this p;atform for 5 months and it works great to my mind. Hope this might be helpful.